Recently, with the increase in concern about environmental problems and energy depletion, a solar cell is attracting an increasing attention as an alternative energy source that has abundant energy resources, has no environmental pollution problem, and has high energy efficiency. Solar cells can be classified into a solar heat cell and a solar light cell. The solar heat cell uses solar heat to generate steam necessary for rotating a turbine. The solar light cell uses semiconductor materials to convert solar light into electrical energy.
The solar light cell can be generally classified into a crystalline silicon solar cell and a thin film solar cell. The crystalline silicon solar cell includes a polycrystalline silicon solar cell and a single crystalline silicon solar cell. The thin film solar cell includes an amorphous silicon solar cell. However, since the crystalline silicon solar cell is manufactured using an expensive thick silicon substrate, the crystalline silicon solar cell has a limitation in reducing its thickness. Furthermore, since the silicon substrate is expensive, a price of the crystalline silicon solar cell becomes high.
What is thus being in the spotlight is the amorphous silicon solar cell that can be manufactured at a lot cost because it uses an inexpensive substrate such as a glass substrate and a metal substrate instead of an expensive silicon substrate and minimizes material consumption by depositing a thin film of several microns.
In comparison with the single crystalline silicon substrate or the polycrystalline silicon substrate, an amorphous-silicon thin film has a very short carrier diffusion length due to its inherent characteristics. Therefore, when forming a PN structure using the amorphous-silicon thin film, the efficiency of collecting electron-hole pairs may be very low. Thus, the amorphous silicon solar cell uses a light-converting layer of a PIN structure that has a highly-doped p-type amorphous silicon layer, a highly-doped n-type amorphous silicon layer, and an undoped i-type amorphous silicon layer inserted between the highly-doped p-type amorphous silicon layer and the highly-doped n-type amorphous silicon layer, and the amorphous silicon solar cell generally includes a first electrode, a PIN light-converting layer, and a second electrode, which are sequentially stacked over a substrate.
However, even when the amorphous silicon solar cell uses a transparent glass substrate, it fails to transmit light incident from the glass substrate since the amorphous silicon solar cell becomes opaque by the PIN light-converting layer and the electrodes are formed over the glass substrate. What is thus manufactured is a transparent solar cell as illustrated in FIGS. 1 and 2. As illustrated in FIGS. 1 and 2, a related art transparent solar cell includes a first electrode 20, a light-converting layer 30, an insulating layer 40, and a second electrode 50 that are stacked over a transparent substrate 10, and has a through hole 60 formed from the substrate 10 to a pre-determined region of the second electrode 50, thus transmitting light from the bottom of the transparent substrate 10 via the through hole 60. The light-converting layer 30 includes a stack of a p-type amorphous silicon layer 31, an i-type amorphous silicon layer 32, and an n-type amorphous silicon layer 33.
However, when the through hole 60 is formed from the bottom of the transparent substrate 10 to the predetermined region of the second substrate 50, the light-absorbing area is reduced and thus the efficiency of the solar cell is deteriorated.